The temperature dependence of electronic absorption spectra of a family of 5,10,15-triarylcorroles in EtOH solution was studied in the range 288-328 K. Corroles in EtOH existed as mixtures of the free base and deprotonated form, the ratio of which was determined by the donor-acceptor properties of the peripheral substituents. Free corrole bases deprotonated as the temperature rose. Deprotonation obeyed the van′t-Hoff equation with activation energy E a = 2.0 kcal/mol, which was the same within measurement error limits for all studied compounds. The temperature was proposed to affect indirectly the deprotonation by changing the EtOH dielectric constant. The decreasing dielectric constant with increasing temperature shifted the acid-base equilibrium toward formation of the deprotonated form.
The temperature dependence of absorption and fl uorescence spectra of 5,10,15,20-tetrakis-(4-sulfonatophenyl)porphyrin in weakly acidic aqueous solutions was studied in the range 288-333 K. The fraction of molecules in the free-base form in the ground (S 0 ) and lower excited singlet states (S 1 ) was found to increase as the temperature increased because the fraction of molecules in the doubly protonated form decreased. The deprotonation was caused by a shift in the acid-base equilibrium in the macrocycle core due to decreases in pK a (S 0 ) and pK a (S 1 ) with increasing temperature. The difference pK a (S 1 ) -pK a (S 0 ) < 0 for solution temperatures >293 K with pK a (S 1 ) -pK a (S 0 ) > 0 for T < 293 K. The activation energies of deprotonation in the S 0 -and S 1 -states for T > 293 K were E a = 5.0 and 3.4 kJ/mol; for T < 293 K, they increased to 20.3 and 56.2 kJ/mol. These differences were explained by different specifi c solvation of the tetrapyrrole macrocycle in the ground S 0 -and lower excited S 1 -states because of a change in the ratio of two forms of water. So-called form A with disordered H-bonds dominated at higher temperatures. The fractions of forms A and B, which possessed a strongly ordered H-bond system, were comparable as the temperature decreased. Stabilization of the porphyrin free base prevailed at high temperatures; of the doubly protonated form, at low temperatures.
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